Support for TensorFlow-Slim

[tyarkoni]

See this. We currently have an Extractor for a pretrained Inception V3 model taken from the TF tutorial docs, but it makes more sense to generalize that to support all slim models, which seem like they have a fairly standard interface.

[qmac]

Looks like whats in the public tensorflow is lagging a little behind (maybe by design) what is in the models/slim repo. Seems like the only way to get full functionality is to clone the repo, which is not ideal for us. We may be able to work something with what's available, just requires more work.

[qmac]

Potentially useful: https://github.com/tomrunia/TF_FeatureExtraction